The signal coming from your video card contains three colour elements: red, green and blue. If the video cable is loose and one of its pins is not connecting then the monitor will lose a colour component, turning the screen yellow or, in your case, purple. Make sure that your monitor connector is firmly screwed in. Try jiggling the cable. If the colour flickers then the cable or the connection to the monitor is the problem.
The same symptoms can occur if there is a problem with the monitor itself. This would be covered by the warranty. However, many smaller computer resellers will not fix the monitor themselves. Monitors can retain several thousand volts of static charge even after the power is turned off, so they require specialist repair skills. This means it may take some time before you get your monitor back. Obviously, don't try and fix it yourself!
With possible monitor problems, it is worth attempting to eliminate all alternative causes. Though unlikely in this case, the video card in your computer may be at fault. Try a different monitor on your computer, or try your monitor on another computer.
- Roy Chambers